NOT FOR MERGE: Initial rustification of hb_direction_t.

[waywardmonkeys]

Thinking about #90 ...

This is one way that we could use enums and is the result of using --rustified-enum with bindgen.

But looking at this, there's this option as well, which I think is better:

#[repr(u32)]
#[repr(C)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum Direction {
    /// Initial, unset direction.
    Invalid = 0,
    /// Text is set horizontally from left to right.
    LTR = 4,
    /// Text is set horizontally from right to left.
    RTL = 5,
    /// Text is set vertically from top to bottom.
    TTB = 6,
    /// Text is set vertically from bottom to top.
    BTT = 7,
}
pub type hb_direction_t = Direction;

Since harfbuzz has a stable API, we don't have to always run bindgen and it might be nicer to have an API that is what we'd actually want in Rust in terms of naming. (And then fixing up other issues in a harfbuzz crate.)

Thoughts?

---

This change is 

[waywardmonkeys]

Worth noting that by allowing ourselves to diverge from the bindgen output, we also can improve the documentation.

[jdm]

I'm in favour of the snippet you describe.

[waywardmonkeys]

@jdm Okay .. once #99 and #102 land, I can update this PR to include all enums and in the style pasted above.

[waywardmonkeys]

@mbrubeck Any thoughts on this?

[waywardmonkeys]

@jdm I've updated the enums that aren't flags now ... what do you think? Still no word from @mbrubeck ...

[mbrubeck]

I'm worried that this is unsafe. For example, if a newer version of Harfbuzz adds new values to the hb_script_t enum, and I use it with a Rust program that calls hb_buffer_get_script from this version of the harfbuzz-sys crate, then it could return an invalid value, which would result in undefined behavior.

[jdm]

True. I guess there's a reason to separate higher-level bindings that use types like enum and provide fallible conversions between the lower-level representation.

[waywardmonkeys]

Okay. I've been thinking about this some and I'm going to close this one and propose something else.